Assisted-listening devices, e.g. hearing aids or the like, should be capable of operating in, and being adaptable to, several environmental conditions. For example, the assisted-listening device should be capable of automatically selecting amongst various audio sources, e.g. telecoil, microphone or auxiliary. There are several possible magnetic field sensors available to measure magnetic fields or B-fields produced by electric currents or the earth's magnetic field, such as, for example a semiconductor magnetic field sensor, a standard Hall effect sensor, or the like. These sensors are can be utilized as detectors for the static B-field of a telephone handset. The silicon external B-field detectors may include: a lateral bipolar magnetotransistor (LBMT), a split-drain MAGFET, or a micro-electromechanical system (MEMS) type device. One commercially available hearing aid utilizes a magnetic reed switch to provide magnetic field detection and automatic transducer mode selection. Unfortunately, there are a number of limitations associated with utilizing the magnetic reed switch. Frequently, the reed switch lacks the sensitivity to operate with many types of telephones and often requires placing an external magnet onto the telephone handset earpiece.
Additionally, the reed switch requires use of a portion of the communication device, such as a very limited space within the hearing aid. Furthermore, the reed switch may be susceptible to damage or performance changes if the hearing aid is dropped or subjected to extremely high magnetic fields—thus undermining the effective reliability of the assisted-listening system. Another shortcoming involves the added costs that are incurred to implement the reed switch into the assisted-listening system due to the additional components and manufacturing effort required.
The LBMT sensor is used to detect magnetic fields that are parallel to the surface of a semiconductor, and is a very sensitive silicon magnetic field sensor device typically having a sensitivity of 100% change in lateral collector current per Telsa. Most conventional LBMT devices require a fairly large, eg., several mA, extrinsic base current to flow so that an enhanced Lorentz force on the majority carriers making up the high extrinsic base current can establish an electric field to divert the laterally diffusing minority carriers.
Accordingly, it would be advantageous to have a LBMT and a method for making the LBMT with increased sensitivity and low power dissipation to operate in many low-power portable applications. It would be of further advantage that the LBMT utilize less power for detecting the presence of a static external magnetic field—often referred to as a B-field commonly associated with a telephone handset, or the like.